The present invention relates generally to superconducting rotating assemblies, and more specifically the invention pertains to a passive superconducting bearing system and process.
The recent discovery of high temperature superconductors (HTSCs) has brought renewed interest in levitation forces produced when magnetic fields are applied to superconductors. When a magnet approaches a superconductor, the superconductor develops a magnetization which tends to repel the magnet. This effect is frequently demonstrated by researchers when a small magnet is levitated above a sample of a high temperature superconductor that is cooled in a dish of liquid nitrogen.
Earnshaw (1842) theoretically showed that a body cannot be stably supported using only permanent magnets--at least one axis must be constrained either non-magnetically or by an active magnetic system that includes closed-loop feedback. This fundamental limitation does not apply to superconductors and other materials which exhibit diamagnetism, i.e., are repelled by a permanent magnet. When a superconductor is cooled below its transition temperature T.sub.c, it will exhibit diamagnetism. As a result, superconducting materials may be incorporated into bearings that require no active feedback elements to achieve stability. The task for providing a passive bearing system for superconductive rotating assemblies is alleviated to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,159,219 issued to Chu et al; PA1 U.S. Pat. No. 5,117,139 issued to Flom et al; PA1 U.S. Pat. No. 4,939,120 issued to Moon et al; PA1 U.S. Pat. No. 4,886,778 issued to Moon et al;
The patents identified above relate to superconducting rotating assemblies. In particular, the Chu et al patent describes a bearing assembly comprising a rotating member and at least one stationary member. A first magnet is mounted on the rotating member, and a second magnet is mounted on the stationary member. The superconductor is located such that it is stationary with respect to one magnet, but in motion relative to the other magnet. This can be accomplished by mounting the superconductor and the first magnet on the rotating member so that they are stationary with respect to each other, and mounting the second magnet alongside the rotating shaft so that the second magnet and the superconductor are in relative motion. The thrust and stability of the bearing system is increased by increasing the magnetic field of the second magnet.
The Flom et al patent is directed to a superconducting rotating assembly. The assembly comprises a rotating member having a magnet at each extremity, and a bearing made of material exhibiting superconducting properties. The bearing is formed as a recess in the superconducting material in the form of a cylindrical, closed-end orifice. The bearing exerts levitation forces on the magnets at each extremity of the rotating member. The levitation forces can be controlled by constructing the bearing from two different types of superconducting materials, or by heating the bearings.
The Moon et al '120 patent relates to a superconducting rotating assembly which includes a floating unsupported rotor. The assembly includes first and second bearings comprised of a material exhibiting superconducting properties. The rotor includes a magnetic pole at each extremity, with each pole resting in a bearing. A temperature bath is provided for maintaining the bearings at a predetermined temperature. Each magnet pole is thereby levitated and adapted to rotate in a non-contacting position by the field and pinning effects generated by the associated bearing.
The Moon et al '778 patent describes the same assembly as the Moon et al '120 patent except that the superconducting material in the Moon et al '778 patent does not totally enclose the circumference of the rotor. Although these patents relate to superconducting rotating assemblies, they do not describe such an assembly where the superconductor magnet interaction generates a restoring force as the shaft is transversely displaced, and where a gas jet is directed towards one end of the shaft to counteract unbalancing axial forces.
There have been recent attempts to develop passive superconducting bearings fabricated from HTSC materials. For example, Moon describes rotating assemblies employing superconducting bearings. For the present invention, a bearing system was developed that was used to rotate a magnetic rotor to a rotational speed of 450,000 rpm, with a peripheral velocity of 150 m/s. A fundamental limitation of these concepts is the lack of means to support the shaft while the superconductor is cooled below its T.sub.c. Because of the hysteretic nature of Type II superconductors, a range of equilibrium positions for the shaft exists. A shaft that starts off in mechanical contact with a non-rotating member must therefore be lifted off by some other means be it manually or otherwise. This limits this type of bearing for use in laboratory demonstrations and basic research devices. As such, the bearing has little practical use.
There remains a need to provide a new superconducting bearing to remove this limitation. By employing a special configuration of permanent magnets and supplying the bearing with a small supply of gas, the shaft may stably and precisely positioned at all times.